[CT-3554] Define SourceDefinition contract in dbt/artifacts

[MichelleArk]

Housekeeping

• I am a maintainer of dbt-core

Short description

As part of an effort to move any ArtifactMixin subclasses upstream of other core modules into the top -level dbt/artifacts directory, we need to move Metric, which WritableManifest depends on.

Acceptance criteria

SourceDefinition (and its necessary dependencies) are moved to a new dbt/artifacts directory. dbt/artifacts should have no additional dependencies on dbt.core except for from contracts.nodes which are actively being refactored.

SourceDefinition should be split into data & functional interfaces, defined in dbt/artifacts in dbt/core respectively. Additionally, any inheritance hierarchy between concrete node classes should be flattened as part of this spike.

Should follow initial pattern set in #9379

Impact to Other Teams

N/A - should be a refactor.

Will backports be required?

nope

Context

No response

[QMalcolm]

Also blocked on #9505